1. Field of the Invention
The present invention relates generally to two-mode photonic crystal optical fiber structures and to applications thereof.
2. Background of the Invention
Two-mode optical fibers have wide applications such as mode converters, mode selective couplers, acousto-optic frequency shifters, acoustic-optic tunable filters, wavelength tunable optical switches, add-drop multiplexers and interferometric fiber sensors, and so on. There generally exists a cut-off wavelength in a conventional circularly symmetric single mode step index fiber. For an appropriate operating wavelength below the cut-off wavelength, two-mode operation can be achieved in such a conventional single-mode fiber. In addition, U.S. Pat. No. 4,832,437, entitled “Fiber Optic Inter-Mode Coupling Single Side Band Frequency Shifter” and issued on May 23, 1989, and U.S. Pat. No. 4,915,468, entitled “Apparatus Using Two-Mode Optical Waveguide with Non-Circular Core” and issued on Apr. 10, 1990, both assigned to The Board of Trustees of the Leland Stanford Junior University, discloses the two-mode applications using elliptical core optical fibers and is herein enclosed by reference.
However, disadvantages exist with the two-mode devices using conventional circular or elliptical core fibers in that these conventional fibers generally only support two-mode operation in a very limited wavelength range. To develop two-mode devices suitable for different operating wavelengths over a wider range, different two-mode fibers or fibers with different parameters are required. This inevitably increases the complexity of the fiber design and fabrication process.